Comparative performance of native and non-native strains of Atlantic salmon ( Salmo salar L.) ranched from the River Bush, Northern Ireland

To evaluate proposals to use commercially farmed Atlantic salmon strains for stocking and ranching purposes, parr from two non-native origin, multi sea winter (MSW) strains (A and B) were brought to the River Bush, Northern Ireland, reared and released after imprinting, together with fish of River Bush ranched strain derived from the native population. All fish released carried a microtag and secondary external mark to facilitate identification in salmon fisheries and upon return to the river. Availability of total adult trapping facilities on the River Bush ensured that returning non-native fish were removed and not allowed to spawn. Non-native groups displayed lower return rates as one sea winter (1SW) fish to the River Bush compared to the native strain, attributable partly to higher exploitation in homewater fisheries around the Irish coast, but also to lower pre-exploitation survival. A small number of strain A fish returned to the River Bush as two sea winter (2SW) adults, indicating some retention of age at maturity characteristics. However, all strain B and River Bush strain returns were as 1SW fish. Non-native fish were recaptured in high seas fisheries at West Greenland and Faroes, whereas no R. Bush strain were reported from those fisheries. Although returning 1SW fish from strain A were significantly larger at capture, total ranching yield was only a third of the R. Bush strain. Differences in performance among strains are discussed in relation to marine migration, exploitation patterns and genetic factors.